feat(uat-batch-4): a11y form primitives + click-to-preview + EOI empty-state + lint guards

- FieldError primitive (role=alert, aria-live) — used by Wave 3
  form-error UX work.
- FieldLabel primitive (Label + Info-tooltip slot) — foundational for
  the platform-wide admin-settings tooltip audit.
- ESLint guard against em-dash in user-facing JSX text inside
  src/components + src/app (warning, not error; 111 existing instances
  flagged for follow-up sweep).
- FileGrid card body becomes click-to-preview button (was hidden under
  a kebab); aria-label per row; kebab keeps Download/Rename/Delete.
- DocumentList: title cell on rows with signedFileId opens
  FilePreviewDialog; kebab gains Download action (was missing
  per UAT). Single FilePreviewDialog instance lifted to the parent.
- DocumentList type extended with signedFileId.
- EOI empty state: third ghost button "Mark signed without file"
  wired to existing MarkExternallySignedDialog (parity with
  reservation tab).
- Watcher empty-state padding fix on document-detail.

tsc clean. 1419/1419 vitest. lint clean on touched files.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

eslint.config.mjs

@@ -35,6 +35,24 @@ const eslintConfig = [
       'react-hooks/incompatible-library': 'off',
     },
   },
+  {
+    // User-facing copy in src/components and src/app should never use
+    // em-dashes (—) in JSX text. The user reads em-dashes as a
+    // tell-tale "AI-generated" marker; we prefer periods, commas, or
+    // simple hyphens. Code comments / audit-log strings / templates
+    // outside these directories are exempt.
+    files: ['src/components/**/*.tsx', 'src/app/**/*.tsx'],
+    rules: {
+      'no-restricted-syntax': [
+        'warn',
+        {
+          selector: "JSXText[value=/\\u2014/]",
+          message:
+            'No em-dash in user-facing JSX text. Use period, comma, or hyphen instead.',
+        },
+      ],
+    },
+  },
   {
     // Tests assert response shape via expect() — narrowing every
     // `res.json()` to a structural type adds boilerplate without catching